Round receiver scope base screws

[Steven69]

I have one of what I suppose is a "contract" L579 receiver which has the "gunsmith" Leupold/Redfield scope bases machined to fit the action. I lost two of the base screws (the barrelled action is in the white to be sent off for BlackNitride finishing). They are #6-40 fillister head hex socket screws with a head diameter of 0.192" going into counterbored holes in the bases measuring 0.195" in diameter (screw OAL is 0.443" and threaded shank is 0.323"). The bases were machined so many years ago that I can't recall the details any more but it seems like the screws were not modified, except perhaps for length, by the gunsmith who machined the bases for me.

All of the #6-40 fillister head screws I've found have larger diameter heads, like 0.22", so maybe the screws I have had the heads reduced somewhat. I will have to wait till tomorrow to contact Brownells to see what they say.
Are these likely standard screws that were just machined for length, or do the head/counterbore dimensions sound special as well? I'm afraid the gunsmith who did the work originally (back in the late 80s or early 90s) probably isn't around to check with.

Thanks!

 

[icebear]

There are fillister head and cheese head screws made with smaller diameter heads. Check a specialty fastener supplier for "narrow-head" screws. You could also chuck a standard size screw in your power drill and turn it down with a mill file. It's funky but I've done it.

 

[Steven69]

Thanks. Turns out they are #6-48s; ordering some from Brownells. They said the head diameter is 0.197", so maybe I won't have to grind them after all.

 

[Steven69]

When I grind them to length, should the threads go the full threaded depth of the receiver bridge? The screws that came with the bases originally (two of which I lost, which occasioned this post) only go about halfway into the bridge (bridge thickness is about 0.340"; original screw threaded length into bridge is about 0.172").

I thought if I ground the new ones flush with the receiver inner diameter there would at least be less space (a really tiny amount, admittedly) for dirt to collect.

 

[paulsonconstruction]

When I grind them to length, should the threads go the full threaded depth of the receiver bridge? The screws that came with the bases originally (two of which I lost, which occasioned this post) only go about halfway into the bridge (bridge thickness is about 0.340"; original screw threaded length into bridge is about 0.172").

I thought if I ground the new ones flush with the receiver inner diameter there would at least be less space (a really tiny amount, admittedly) for dirt to collect.

Rule of thumb for screws that attach bases or sights to a rifle is 3 threads minimum. More than that is obviously better. As long as they don't interfere with the bolts operation or bottom out on the barrel threads, they can't be too long.